Petroleum recovery by in situ combustion



FIFWIZ J. ORKISZEWSKI PETROIJEUM RECOVERY BY IN SITU COMBUSTION Sept 10, 1968 F1 l ed Se FUEL CONTRIBUTION OF NATURAL RESIDUUM (=F I8 20 22 24 26 28 3O 32 34 36 GRAVITY OF RESERVOIR OIL, API

n n O 0 FUEL CONTRIBUTION OF RESIDUUM FORMED IN-SITU (=F I 80 I00 INITIAL FORMATION TEMPERATURE, F.

JOSEPH ORKISZEWSKI INVENTOR.

ATTORNEY iinired 3,400,760 PETROLEUM RECOVERY BY IN SlTU COMBUSTiON Joseph Orkiszewski, Houston, Tex., assignor to Esso Production Research Company, a corporation of Deiawnre Filed Sept. 14, 1966. Scr. No. 579,290 Claims. (Ci. 166-4) ABSTRACT OF THE DISCLOSURE In an in situ combustion process for the recovery of oil, the available fuel is changed to more nearly equal the minimum fuel requirement for the process. The API gravity of the reservoir oil and the temperature of the reservoir are used to determine the total available fuel This invention relates to the recovery of petroleum from porous subsurface reservoirs by in situ combustion. A

method is provided for controlling the amount of oil consumed as'fuel during petroleum recovery by in situ combastion. In accordance with one aspect of the invention, fuel availability is controlled by adjusting the reservoir temperature ahead of the combustion front. The selection of a suitable temperature is made in accordance with a novel correlation based in part upon the relationship between fuel availability and the API gravity of the reservoir crude.

-In accordance with a second aspect of the invention, fuel availability is controlled by displacing or diluting the reservoir crude with an oil having a different API gravity, selected in accordance with a novel correlation 7 based in part upon the relationship between fuel availability and original reservior temperature. Frequently, it

will be desirable to control fuel availability by utilizing" a combination of the above embodiments; that is, by adjusting the reservoir temperature ahead of the combustion front, and also by injecting an oil of different API gravity.

Petroleum recovery by in situ combustion' usually' in-" volves the injection of air or other oxygen-comprising gas at an input well, where a combustion front is initiated, and the production of displaced crude from a spaced output well. In order to achieve anefiicient displacement of the crude, it is essential to propagate the combustion front outward from the injection well. However, substantially all the fuel available to support combustion must be consumed in any given region of the reservoir before the front can be advanced to each successive adjacent region more remote from the injection well.

The economic success of the operation therefore depends critically upon the amount of fuel available to support combustion. it is generally recognized that the concentration of naturally available fuel in some reservoirs is insufficient to support a combustion front. In such cases, combustion can be sustained; if at all, for a relatively short distance from the injection well, thereby preventing completion of the recovery process. in other reservoirs, the availability of excess fuel is known to prevent economic utilization of the method, since consumption of the excess fuel sharply increases the cost of the operation, due to the need for compressing and injecting a corresponding extats aterrt ce 3,460,766 Patented Sept. 10, 1968 cess of air or other oxygen-containing gas. Moreover, the excess consumption of oil as fuel obviously limits the amount of oil which can be displaced toward the production well or wells for recovery.

Methods are known for calculating the minimum fuel 1 concentration required in a given reservoir to sustain combustion. See, for example, H. S. Ramey, "Transient Heat Conduction During Radial Movement of a Cylindri cal Heat Source-Application to the Thermal Recovery Process," Trans. AIME (1959), vol. 216, page 115; and C. Chu, Two-Dimensional Analysis of a Radial Heat Wave," Jounral of Petroleum Technology, October 1963, page 1137. By comparing the theoretical minimum fuel requiremcnt with an experimental determination of the actual fuel availability in a given reservoir, the industry has been capable of reaching a qualitative determination that the available fuel is either insufficient or is excessive.

It has also been recognized that a generally inverse relationship exists between total fuel consumption and the API gravity of the reservior oil. it has been therefore proposed to inject a residual oil having an intermediate API gravity of 30-40 into the formation prior to the initiation of in situ combustion, as a means of optimizing fuel availability. Such a proposal clearly assumes that oils having an API gravity of less than 30 are generally characterized by the deposition of excessive fuel, and that oils having an API gravity above 40 generally deposit inadequate concentrations of fuel to sustain combustion. It has more recently been observed, however, that an oil having a given AIPI gravity may possess optimum fuel-depositing character in one reservoir but not in another. Moreover, in a given reservoir a given oil may he optimum under certain conditions, but not for other conditions.

In accordance with the present invention, it has been found that the initial reservoir temperature has substantiaily as much influence upon the fuel-depositing character of a given oil as does its API gravity. For example, the

total fuel consumption which may result from the in situ combustion of an oil having a gravity of 30 API will vary from 1.0 lb. per lbs. of rock up to at least 1.4 lbs. per 100 lbs. of rock, depending upon the initial reservoir temperature. In a given reservoir, 1.0 lb. of fuel per 100 lbs. of rock may be adequate to sustain combustion, in which event 1.4 lbs. of fuel per 100 lbs. of rock would range far above the economically feasible limit. It therefore becomes apparent that the mere selection of an oil having a certain API gravity may give little or no assurance of economic feasibility.

" At least a partial understanding of the effect which initial reservoir temperature has upon fuel availability may be had from a consideration of the nature of the fuel deposit itself. Although the fuel consumed in an under ground combustion operation is commonly referred to as coke, recent data have indicated that the ratio of hydrogen to carbon in the burned material is considerably higher than is characteristic of a true petroleum coke. Since the hydrogen-to-carbon ratios range from' 1.4 to 1.6, the fuel is probably an extremely viscous, non-distillable tar or other non-distillablc residuum.

A portion of such fuel deposit is derived directly from natural residuum contained in the reservoir oil. But the reactions occurring ahead of the combustion front. Frcquently, the fuel deposited as a result of low-temperature oxidation reactions exceeds the amount of fuel contributed by naturally occurring residuum.

In a forward combustion process, the oxygen which reacts ahead of the combustion front to produce low-temperature oxidation products must first pass through the combustion front. As a practical matter, a substantial proportion of hte oxygen which passes through the combustion front may simply bypass or channel around the regions of highest combustion temperature. Thus, the amount of oxygen which is available to react at low temperature increases as the efficiency of the high-temperature combustion process decreases. Conversely, the higher the temperature at the combustion front, the more efficient will be the consumption of oxygen at the combustion front, and a smaller proportion of the injected oxygen will pass through.

The temperature at the combustion front is in turn infiuenced by the initial reservoir temperature. The temperature of the combustion front will be increased by an increase in the original reservoir temperature, which in turn causes a more efficient consumption of oxygen at the high-temperature front. Accordingly, less oxygen will be available ahead of the combustion front to form low-temperature oxidation products. Conversely, a low initial reservoir temperature leads to an increased fuel availability due to increased deposits of low-temperature oxidation I products.

' temperature.

The correlation of FIGURE 1 was developed from a combination of laboratory data and a theoretical analysis of the mechanisms discussed above. Various attempts to experimentally correlate total fuel consumption with the API gravity of the reservoir oil have reliably shown that a wide range of values may be obtained for the gross fuel availability when recovering a given reservoir crude from representative cores of a given reservoir. It was reasoned that the fuel contribution from the natural residuum contained in a given crude must remain constant from one run to the next. Therefore, the observed variation in gross fuel consumption is wholly attributable to variations in the amount of residuum formed in situ by low-temperature oxidation ahead of the combustion front.

It was further reasoned that since first-order kinetics govern the low-temperature oxidation reactions, 21 straightline plot on Cartesian coordinates should result from a correlation of total fuel concentration with the fraction of total oxygen consumed by low-temperature oxidation. Accordingly the fraction of total oxygen consumed in lowtemperature oxidation was calculated from the following equation:

fx= f,.fu where:

i fraction of oxygen consumed in low-temperature oxidation f =fraction of oxygen consumed in complete combustion f =fractin of oxygen not consumed The experimental value for f was used, and i was calculated from classical mass transfer theory. The various observed total fuel concentrations for a given crude were then plotted versus the calculated values for f The resulting straight-line plot was extrapolated to obtain the total fuel concentration corresponding to an of zero. Such a value for total fuel concentration is simply the fuel contributed by natural residuum (F F for several additional crudcs was determined in like manner, and the results lotted versus API gravity to give the correlation of FIGURE 1.

FIGURE 2 was then obtained from published reports of field results by subtracting F in each instance from the 4 reported values for total fuel concentration and plotting the net values (F versus initial formation temperature.

In order to appreciate the significance of these correlations, assume for example that it has been determined in a given reservoir that the minimum fuel concentration required to sustain and propagate a combustion front is 1.1 lbs. of fuel per lbs. of reservoir rock. Moreover, assume that the API gravity of the oil in place is 18 API and that the initial reservoir temperature is 100 F. Thus, from FIGURE 1 the fuel concentration contributed by the natural residuum is seen to equal 0.73 lb. per lt'Q lbs. of rock, and from FIGURE 2 the fuel contribution resulting from low-temperature oxidation products formed ahead of the combustion front is seen to equal 0.66 lb. per 100 lbs. of rock. The total fuel available is therefore 1.39 lbs. per.l00 lbs of reservoir rock, which corresponds to an access of .29 lb. per 100 lbs. of rock.

In order to reduce or eliminate the excess available fuel, an increase in the oil gravity or an increase in reservoir temperature, or both, should be considered. For example, an increase of API gravity to 26" and an increase in reservoir temperature to l45 F. would result in a contribution of 0.55 lb. of fuel per 100 lbs. of rock from natural residuum, and an equal contribution from the low-temperature oxidation products. thereby totalling 1.1 lbs. of fuel per 100 lbs. of rock, which is the minimum fuel concentration required to sustain combustion, as stated above.

Using the correlations of FIGURES 1 and 2 together with the following equation, total fuel concentration, P can be expressed as pounds of fuel per cubic foot of rock:

where:

F fuel due to natural residuum as determined by FIG- URE 1 F =fuel generated by low-temperature oxidation determined in accordance with FIGURE 2 =porosity of the reservoir rock g=sand grain density in pounds per cubic foot, typically 163.5 lbs. per cu. ft. for siliceous sands In accordance with, one embodiment of the invention, the fuel availability is adjusted or controlled by changing the reservoir temperature, without changing the gravity of the reservoir crude. In order to determine whether the existing reservoir temperature is too high or too low, it is necessary first to determine the minimum fuel concentration required to sustain combustion and the API gravity of the reservoir oil. This information is ascertained by conventional means. Then, by reference to FIGURE 1, the fuel contribution due to natural residuum in the reservoir oil is determined. This value, F and the minimum required concentration, F,,, are then substituted in the following solution of the above equation:.

The computed value for F is then used to determine the theoretical optimum reservoir temperature by reference to FIGURE 2. If the optimum is higher than the existing reservoir temperature. then of course the reservoir temperature must be raised. If the optimum falls below the existing temperature. the reservoir should be cooled in advance of the combustion front.

A second embodiment of the invention relates to situations which call for displacement or dilution of the original reservoir crude with an oil of different API gravity, in order to adjust the fuel availability ahead of-the combustion from. In such circumstances, it is first necessary, as before. to determine the minimum total fuel required to sustain combustion by conventional calculation techniques. The reservoir temperature is determined, and this value is used together with the correlation of FIGURE 2 to determine that component of fuel availability which corresponds to the given reservoir temperature. The minimum contribution required from the natural residuum is then calculated by substitution in the following form of the above equation:

The computed value for F is then used to determine the optimum API gravity by reference to FIGURE 1. If

the optimum is higher than the gravity of the natural crude, then an oil of higher gravity is selected for injection. If the optimum is lower than the gravity of the crude, then an oil of lower gravity is selected for injection.

Once it is determined in accordance with the first embodiment above that the reservoir temperature must be adjusted in advance of a combustion front, the temperature change can be brought about by any of various means. For example, in order to raise the reservoir temperature, it will generally be preferred to inject hot water or steam. The injection of other hot, non-oxidizing fluids is also contemplated-eg, natural gas, nitrogen, CO and combustion products, if sufliciently oxygen-free. The presence of any oxygen or oxidizing gases in the fluid used to raise the reservoir temperature would, to some extent, defeat the purpose of raising the reservoir temperature, since an increase in the reservoir temperature is to reduce the formation of low-temperature oxidation products, whereas the use of an oxygen-containing gas would cause the opposite result. It is also possible to raise the reservoir temperature by the injection of exothermically reactive chemicals to release heat energy upon contact with reservoir fluids.

For example, sulfur trioxide reacts both with water and with the aromatic constituents of reservoir oil to raise the l'CStil'VOll' temperature.

In order to cool the reservoir. when it is desired to increase fuel availability, the injection of cold water in advance of a combustion front will generally be suitable. The injection of water or other fluids at ambient surface temperature is efiective in reservoirs having an initial temperature substantially above ambient surface temperature. Since a lowering of the reservoir temperature is for the purpose of increasing fuel availability, the'injection of cold air or other oxygen-comprising gases is beneficial, since it will accomplish both the cooling of a reservoir and some formation of low-temperature oxidation products even before combustion is initiated.

As a further means of controlling the fuel contribution of residuum formed in situ, oxidation inhibitors or catalysts may be injected ahead of the combustion front. For

example, if it is desired to reduce the fuel contribution g from low-temperature oxidation. an inhibitor such as pyrocatechol, aniline, or hydroquinone is injected. If it is desired to accelerate the formation of low-temperature oxidation products, a catalyst such as cobalt naphthenate or a peroxide is injected. The correlation of FIGURE 2 is altered somewhat by the addition of inhibitors or catalysts. The curve as plotted would be shifted slightly upward by a catalyst and downward by an inhibitor.

In the event an oil of suitable API gravity is available for the purpose of injection to displace or dilute the resorvoir crude. it"is especially preferred that the temperature of the injected oil be controlled in order to obtain the further benefit of adjusting the reservoir temperature. That is. when injecting an oil of APl gravity higher than the reservoir crude for the purpose of reducing fuel availability, a further reduction in fuel availability is readily achieved by heating the injected oil to raise the reservoir temperature. Conversely, where the injected oil has an API gravity lower than that of a reservoir crude for the purpose of increasing fuel availability, a further increase in fuel may be obtained by reducing the temperature of the injected oil below that of the reservoir whereby a reduction in reservoir temperature is achieved. The injected oil may be a crude pertoleum oil, a processed residuum from the distillation of crude oil, or a distillate therefrom. Such an oil is injected at the input well for the purpose of displacing or diluting the natural crude oil because of its unsuitable fuel-depositing character.

The volume of such substitue oil to be injected will vary, depending upon the nature of the formation, the oilin-place, and the particular choice of injected oil. Typically, a volume corresponding to no more than 10% of the pore volume occupied by the natural crude will be required. However, it is contemplated that from 5% to about 20% will be suitable.

In many instances it will not be practical to fully compensate for large excesses in fuel availability. That is, in some reservoirs it may not be practical to change the initial temperature more than l0-20 F. and, similarly, it may be that the onlyoils available for the purpose of adjusting API gravity will not differ sufi'iciently from that of the original crude to permit the attainment of a fuel availability which approximates the minimum to sustain combustion. Nevertheless, any substantial effort to approach the theoretical minimum by coordinating the API gravity and reservoir temperature in accordance with the novel correlations of the present invention is contemplated as being within the scope of the appended claims.

What is claimed is:

l. A. method of controlling the available fuel in an in situ combustion process having a minimum fuel requirement and conducted in an oil-bearing substerranean reservoir penetrated by a wellbore comprising:

(a) determining the total available fuel existing in the reservoir from the reservoir properties consisting of the API gravity of the reservoir oil and the initial reservoir temperature;

properties necessary to make the total available fuel equal to the minimum fuel requirement; and

(c) injecting a fluid into the reservoir through the wellbore to change at least one reservoir property so that the available fuel is more nearly equal to the minimum fuel requirement.

2. The method as defined by claim 1 wherein the total available fuel per pounds of reservoir rock is the summation of the fuel contribution of the natural residuum as determined from the API gravity of the reservoir oil and the correlation of FIGURE 1 and the fuel contribution of residuum formed in situ as determined from the initial temperature of the reservoir and the correlation of FIGURE 2.

3. The method as defined in claim 1 wherein the amount of change in reservoir properties is determined from the correlations of FIGURES l and 2.

4. The method as defined in claim 1 wherein the injected fluid is an oil having an API gravity higher than that of the reservoir oil and a temperature higher than the reservoir temperature.

5. The method as defined in claim 1 wherein the injected fluid is an oil having an API gravity lower than that of the reservoir oil and a temperature lower than the reservoir temperature.

6. The method as defined in claim 5 wherein the injected fluid has a temperature greater than the reservoir temperature.

7. The method as defined in claim 6 wherein the injected fluid is steam.

8. The method as defined in claim 1 wherein the injected fluid has a lower temperature than the initial reservoir temperature.

9. The method as defined in claim 8 wherein the fluid is water.

10. A method for the recovery of oil by an in situ combustion process having a minimum fuel requirement from a substerranean reservoir penetrated by a wellbore comprising:

(a) measuring the API gravity of the reservoir oil;

(b) measuring the initial reservoir temperature;

1-! Hiya-1 e (c) determining the total available fuel in the reservoir from the API gravity of the oil. the initial reservoir temperature and the correlations of FIGURES l and 2;

(d) injecting a fluid into the reservoir to change the available fuel to more nearly equal the minimum fuel requirements;

(c) injecting an oxygen contaiing gas into the reservoir to initiate combustion; and

(f) recovering oil from the reservoir.

References Cited UNITED STATES PATENTS Dew et a]. 166-11 Alexander et al l66ll Prats 166ll Chalenever 166-ll Hardy 16611 0 STEPHEN J. NOVOSAD, Primary Examiner. 

